Colicin E7

Colicin E7 is a type of Colicin, a bacteriocin made by E. coli which acts against other nearby E. coli to kill them with its DNase Activity; it digests the cell's genome in specific locations, ultimately leading to the death of the cell.

Synthesis and release
After synthesis inside the E. coli cell, the colicin binds its Colicin Immunity Protein, Im7, to its nuclease domain, to prevent the host cell from being killed by its activity. This immunity protein is released only on binding to the outer membrane of the target cell.

Mechanism of uptake
The receptor binding domain of ColE7 binds to the BtuB vitamin B12 receptor on the outer membrane of the target cell. This causes the dissociation of the immunity protein from the cytotoxic domain of the colicin, allowing it to target the cell when it enters. Binding BtuB triggers the recruitment of OmpF and the TolQRAB complex, to bind to the translocation domain and translocate the colicin across the membrane. The mechanism by which this occurs has not yet been identified. This structure shows the translocation domain of ColE7.

Killing Activities
Colicin E7 has an endonuclease domain that degrades the DNA of the targeted cell, containing an H-N-H motif. This structure shows this domain bound to a DNA octamer.

The endonuclease domain is likely to use Zn2+ in the DNA cleavage, although other literature does not support this. For cleavage of single stranded DNA or RNA ColE7 can either use Ni2+ or Co2+. The bound ion binds to the second histadine residue in the HNH motif and stabilises the reaction.